This invention relates to displacement measuring apparatus and, more particularly, to a new and improved fiber optic displacement measuring apparatus having greatly increased working distance and measuring range capabilities.
Fiber optic displacement measuring devices utilizing a bifurcated fiber optic bundle in combination with source and a photo detector or other suitable light receiver for measuring displacements are known. One such device is described in Kissinger U.S. Pat. No. 3,327,584, and comprises a fiber optic bundle divided at one end into separate arms. One arm contains transmitting fibers and the other arm contains receiving fibers. The other or common end comprises a common bundle of transmitting and receiving fibers. When placed with the face of the common end of the fiber optic bundle in close proximity to a reflective target, light from the transmitting fibers impinges on the target and is reflected back to the receiving fibers where it is conducted to a photosensor or other light receiver. The amount of reflected light is related to the distance between the end face of the fibers in the common bundle and the surface of the target, the diameter of the fibers, the numerical aperture of the fibers, the geometrical distribution of the transmit and received fibers, the total number of fibers, and the reflectivity of the target surface. Thus, as the gap between the bundle and the target increases, starting from a position of direct contact of the end face of the bundle with the surface of the target, greater amounts of light impinge on the receiving fibers in a substantially linear relationship to the displacement of the target. The amount of reflected light peaks and then decreases, however, once the bundle end face and target are separated by a distance on the order of 5 thousandths of an inch (5 mils).
Improvements over devices of the type just described are also known in the prior art. The apparatus described in Kissinger et al U.S. Pat. No. 3,940,608 includes a sensing head having a pair of projection lens mounted so that the end face of the common bundle is adjacent one end of the lens, the other end of the lens adapted to be positioned adjacent the object whose displacement is to be determined. The lens is operative, at a predetermined distance from the target, to focus the image of the end face of the common bundle onto the object and to refocus this image back on to the end face so that light transmitted from the fibers associated with the light source is returned upon itself. At this precise distance, known as the image focal point, the relative intensity of light detected by the sensor is at a sharp minimum, since the light is being reflected back onto transmitting as opposed to receiving fibers. Displacement of the object either closer or more distant than the image focal point causes defocusing of the image reflected back onto the end face so that more of the transmitted light is reflected onto receiving fibers and transmitted to the light receiving means to provide an output indicative of the target displacement.
The benefit provided by the '608 patent resides in an increase in the working distance (the distance from the sensing head to the surface to be measured) by a factor of about 100, i.e., from thousandths of an inch to tenths of an inch. Moreover, the '608 patent recognizes that by adjusting the gap between the end of the fiber and the lens the working distance may be effectively changed. However, despite the change in working distance, for a given lens configuration, the sensitivity and dynamic range of the structure shown in the '608 patent remains essentially constant.
Moreover, the apparatus illustrated in the '608 patent is a highly sensitive measuring device which can restrict its utility in measuring displacement relative to a surface subject to contamination that may affect the reflective characteristics of the target surface. For example, if a panel is being measured with the highly sensitive device of the '608 patent, a relatively small piece of dirt may cause a false reading due to the relatively small focused spot of light striking the target surface.
The present invention is an improvement over the displacement measuring apparatus of the '608 patent in that it enables displacement determination at greatly increased working distances from a target and over substantially larger dynamic ranges.
In addition, the sensitivity of the displacement measuring apparatus of the invention can be adjusted thereby limiting the likelihood of false readings due to contamination of a target surface in those situations where such contamination is a possibility.